The growth in demand for telecommunication services is increasing at an ever-quickening pace. The majority of the demand is being driven by the explosion in the use of the Internet and a steady stream of new applications being introduced which further increase the demand for increased bandwidth. Currently, a large portion of the Internet traffic is still carried by circuit switched transport facilities. In the case of Metropolitan Area Networks (MANs), most of the traffic is transported over SONET/SDH based networks most of which were originally resigned designed for voice traffic.
The requirements for networked communications within the user community have changed dramatically over the past two decades. Several notable trends in the user community include (1) the overwhelming domination of Ethernet as the core networking media around the world; (2) the steady shift towards data-oriented communications and applications; and (3) the rapid growth of mixed-media applications. Such applications include everything from integrated voice/data/video communications to the now commonplace exchanges of MP3 music files and also existing voice communications which have begun to migrate towards IP/packet-oriented transport.
Ethernet has become the de facto standard for data-oriented networking within the user community. This is true not only within the corporate market, but many other market segments as well. In the corporate market, Ethernet has long dominated at all levels, especially with the advent of high-performance Ethernet switching. This includes workgroup, departmental, server and backbone/campus networks. Even though many of the Internet Service Providers (ISPs) in the market today still base their WAN-side communications on legacy circuit oriented connections (i.e. supporting Frame Relay, xDSL, ATM, SONET), their back-office communications are almost exclusively Ethernet. In the residential market, most individual users are deploying 10 or 100 Mbps Ethernet within their homes to connect PCs to printers and to other PCs (in fact, most PCs today ship with internal Ethernet cards) even though the residential community still utilizes a wide range of relatively low-speed, circuit-oriented network access technologies.
The use of Ethernet, both optical and electrical based, is increasing in carrier networks due to advantages of Ethernet and in particular Optical Ethernet, namely its ability to scale from low speeds to very high rates and its commodity-oriented nature. With the rapid increase in the demand for user bandwidth, and the equally impressive increase in the performance of Ethernet with the LAN environment, the demand for Metropolitan network performance is rapidly increasing. In response, there has been a massive explosion in the amount of fiber being installed into both new and existing facilities. This is true for both the corporate and residential markets.
A problem arises from the fact that conventional Ethernet networks are designed to transfer packets from one location to another asynchronously. Conventional Ethernet networks lack a mechanism for providing clock synchronization and distribution from a centralized clock source location. They are not capable of conveying information synchronously and thus cannot support TDM traffic streams, for example. In an asynchronous network, each node in the network generates its clock independently from all other nodes. Currently, if there is a need to transfer clocking information such as TDM traffic, other network types can be used, such as synchronous networks wherein an accurate clock source is distributed around the network. An example of a synchronous network is the well known Synchronous Optical Network (SONET)/Synchronous Data Hierarchy (SDH) network. In a SONET network, a high quality clock is distributed over the synchronous network. Existing SONET/SDH networks perform clock synchronization and distribution over a TDM based network. A short description of SONET follows.
The synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE. It was developed to replace the Plesiochronous Digital Heirarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors. The more recent synchronous digital hierarchy (SDH) standard developed by the International Telecommunication Union (ITU) is built on experience in the development of SONET. It is documented in standard G.707 and its extension G.708. Both SDH and SONET are widely used today; SONET in the United States and Canada, SDH in the rest of the world.
SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.
Another circuit type used more and more in data networking equipment is 10 Gigabit WAN Ethernet (10 G-WIS). This is similar in rate to OC-192/STM-64, and, in its wide area variant, encapsulates its data using a light-weight SONET/SDH frame so as to be compatible at low level with equipment designed to carry those signals. 10 Gigabit LAN Ethernet, however, does not explicitly provide any interoperability at the bitstream level with other SONET/SDH systems. This differs from WDM system transponders, including both coarse- and dense-WDM systems that currently support OC-192 SONET Signals, which can normally support thin-SONET framed 10 Gigabit Ethernet.
Regarding synchronization of SONET and SDH networks, a SONET NE transports and/or multiplexes traffic that has originated from a variety of different clock sources. In addition, a SONET NE will typically have a number of different synchronization options to choose from, which in some cases it will do so dynamically based on Sync Status Messages and other indicators.
The synchronization sources available to a SONET NE typically include:
1. Local external timing generated by an atomic Cesium clock or a satellite-derived clock by a device located in the same central office as the SONET NE. The interface is typically DS1 with Sync Status Messages supplied by the clock and placed into the DS1 overhead.
2. Line-derived timing whereby a SONET NE can be configured to derive its timing from the line-level, by monitoring the S1 sync status bytes to ensure quality.
3. Using holdover wherein, in the absence of higher quality timing, a SONET NE uses its own timing circuits to time the SONET signal until a higher quality external timing becomes available again.
A disadvantage of using a SONET/SDH network for clock synchronization is the high cost of such networks. A further disadvantage is that having been developed for TDM networks, SONET/SDH networks are not optimized for Ethernet transport which transmits data asynchronously.
Thus, a problem exists in how to transfer legacy TDM traffic over an asynchronous Ethernet network and particularly, how to extract and reconstruct the TDM clock from the received data at the other side. It is important that the clock used at the receive side be traceable to the clock used at the transmitter. The clock at the transmitter side can be provided from an external source, a clock distribution network or from SONET/SDH equipment.
One prior art solution is to break the TDM traffic into several channels and convert the voice data to IP packets using DSP algorithms and then switch this IP traffic over the network. This approach, however, does not address transporting TDM traffic over asynchronous Ethernet networks.
Other schemes attempt to recover the TDM clock by maintaining a buffer and a pointer whereby if the slave clock is following the master clock, the pointer is designed to stay in the mid point of the buffer. Deviations of the slave clock from the master clock cause the pointer to move away from the mid point of the buffer. The location of the pointer is monitored and suitable action taken if it is detected to have moved.
A disadvantage of this scheme, however, is that the jitter and wonder generated is relatively high and typically does not meet common carrier telecommunication standards. In addition, the scheme cannot track the master clock in the event a large number of packets are lost and is overly sensitive to lost and erroneous packets.
Alternatively, an algorithmic approach can be used to transfer a clock. In this clock reconstruction mechanism, synchronous TDM communications traffic is transported over asynchronous networks such as Ethernet networks. The network comprises edge switches in Metropolitan Area Networks (MANs) that transport legacy TDM traffic using a Circuit Emulation Services (CES) module whereby TDM traffic is encapsulated and transported across the Ethernet network where it is de-encapsulated and clocked out to the destination. The input TDM data stream is encapsulated into Ethernet packets and a network timestamp is inserted into the packet. At the destination CES, a local timestamp is generated for each received packet as it is received. The network timestamp is extracted and input along with the local timestamp to a Digital Time Locked Loop (DPLL) which is operative to accurately reconstruct the original transmit TDM clock.
The clock quality output from this mechanism is, however, of medium quality. In addition, it is susceptible to network delays and congestion. A more detailed description of the algorithmic approach can be found in U.S. Pat. No. 7,289,538, entitled “Clock Reconstruction for Time Division Multiplexed Traffic Transported Over Asynchronous Ethernet Networks,” incorporated herein by reference in its entirety.
There is thus a need for a clock synchronization and distribution mechanism operative over an asynchronous network such as Ethernet that overcomes the disadvantages of the prior art. The mechanism should be capable of passing a high quality clock over an Ethernet physical network by making the network synchronous. Preferably, the mechanism is relatively straightforward to implement and is cost effective to make its use practical.